Manufacturing method for high strength steel plate

Abstract

The purpose of the present invention is to provide: a steel sheet that has excellent LME resistance characteristics, high strength, and good moldability and is obtained by a method differing from those of the prior art; and a method for producing the steel sheet.　This high-strength steel sheet contains specific components, has a specific structure, includes Nb in a manner satisfying the relationship between the solid solution Nb amount (Nb sol), the Nb amount (Nbpre) in Nb precipitates having a particle size of less than 20 nm, and the total Nb amount (Nb) included in the steel sheet expressed in formula (1) below, and includes 0.50 mass ppm or less of diffusible hydrogen in steel. Formula 1: (Nbsol / Nb) + (Nbpre / Nb) ≥ 0.40 (in formula (1), Nbsol represents the solid solution Nb amount (mass%) and Nbpre represents the Nb amount (mass%) in Nb precipitates having a particle size of less than 20 nm).

Description

[Technical Field]

[0001] The present invention relates to a steel sheet and a manufacturing method thereof, and more particularly to a high-strength steel sheet excellent in LME resistance and suitable for use as components formed by cold pressing in industrial fields such as automobiles and electrical machinery, and a manufacturing method thereof. [Background technology]

[0002] In recent years, there has been an increasing need to reduce the weight of automobile bodies in order to improve fuel efficiency and protect the global environment. Therefore, the application of high-strength steel sheets to automobile parts is desired. When cold-rolled steel sheets are used for automobile parts, they are typically formed by welding two or more steel sheets together to form the desired shape.

[0003] Spot welding is mainly used to join steel sheets in automotive parts, but it is known that when welding is performed on sheet assemblies that include high-strength steel sheets with a welded zinc-based plating layer (high-strength zinc-based plated steel sheets), cracks are likely to occur in the welds. This is thought to be because zinc-based coating layers generally have a low melting point, and the coating layer becomes liquid during welding, penetrating the grain boundaries of the base steel sheet and reducing the grain boundary strength. This reduction in strength due to liquid metal is generally called liquid metal embrittlement (LME), and cracks that occur due to liquid metal embrittlement are called liquid metal embrittlement (LME) cracking.

[0004] It is also known that even if a high-strength steel sheet is not plated, cracks will still occur in the welded portion of the high-strength steel sheet if the mating material to be welded to the high-strength steel sheet has a zinc-based plated layer. LME cracks that occur in high-strength steel sheets that do not have a plated layer are called transferred LME cracks.

[0005] The aforementioned LME cracking is particularly noticeable when spot welding is performed with an electrode at an angle, where the axis of the welding electrode is not perpendicular to the surface of the steel sheet.

[0006] Therefore, various studies have been conducted to prevent LME cracking of zinc-based coated steel sheets when they are spot-welded, and to prevent LME cracking of uncoated steel sheets.

[0007] For example, Patent Document 1 discloses a technology for realizing a high-strength steel sheet with a tensile strength of 980 MPa or more, a total elongation of 20% or more, and excellent LME resistance by controlling the frequency of coincidence grain boundaries in the surface layer of the steel sheet after a high-temperature tensile test and the thickness of the softened surface layer.

[0008] Furthermore, Patent Document 2 discloses a technology in which oxygen is introduced into the surface layer of a steel slab during continuous casting to form iron oxide, which is a site for the formation of titanium nitride, and this inhibits the bonding of B in the steel with dissolved nitrogen during the subsequent annealing process, thereby promoting the formation of (Fe, Mn)2B in the surface layer region of the steel sheet, thereby realizing a steel sheet that is resistant to LME, has high strength, and is excellent in ductility.

[0009] Furthermore, Patent Document 3 proposes a technique for preventing LME cracking by removing the plating layer from the portion to be welded prior to spot welding. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] International Publication No. 2020 / 184154 [Patent Document 2] International Publication No. 2020 / 225936 [Patent Document 3] International Publication No. 2016 / 159169 Summary of the Invention [Problem to be solved by the invention]

[0011] However, the technology described in Patent Document 1 requires a reduction in Si in order to reduce the frequency of coincidence grain boundaries in the steel sheet surface layer, and it is presumed that it is difficult to impart good formability to steel sheets with strength levels higher than 980 MPa.

[0012] In addition, the technology described in Patent Document 2 improves the LME resistance of the final steel sheet by introducing oxygen into the surface layer of the steel slab during continuous casting, but the introduction of oxygen into the surface layer causes scale to form, which is expected to reduce yield. Furthermore, since the surface layer contains a large amount of hard particles, (Fe, Mn)2B, from the casting stage, it is expected to cause surface cracks during casting.

[0013] Furthermore, the technology described in Patent Document 3 requires a step of removing the plating layer in advance, which increases manufacturing costs. Furthermore, since the plating layer is removed, it is believed that the corrosion resistance of the welded portion decreases.

[0014] Therefore, an object of the present invention is to provide a steel sheet having excellent LME resistance, high strength, and good formability, and a manufacturing method thereof, which is different from the conventional techniques. The steel sheet includes a hot-rolled steel sheet, a cold-rolled steel sheet, and a plated steel sheet such as GA or GI.

[0015] In the present invention, "high strength" means that TS is 980 MPa or more, and "good formability" means that the relationship between tensile strength TS and elongation El shown in the following formula (7) is satisfied. (Equation 7) TS 1.5 ×El≧390000 [Means for solving the problem]

[0016] In order to solve the above problems, the inventors have conducted extensive research into the chemical composition and microstructure of steel sheets, and as a result, have discovered the following by carefully controlling the slab heating conditions, temperature management from hot rolling to coiling, and annealing conditions, and by controlling the state of elements contained in the steel.

[0017] That is, by dissolving Nb in solid solution, or by controlling the particle radius of precipitates so that they do not cause cracks on the steel surface during casting and fall within a certain particle radius range, the solid solution elements present in the steel during welding, or the solid solution elements formed by dissolving the precipitates, prevent the penetration of zinc, thereby improving LME resistance. On the other hand, if the precipitate radius of a specific element is larger than a specified value, the precipitates will not dissolve sufficiently during welding, and not only will the improvement in LME resistance due to the solid solution elements not be expected, but it is also quite possible that new cracks will occur, originating from the coarse precipitates, not only during manufacturing but also after press working and during welding.

[0018] The present invention has been made based on the above findings, and the gist of the present invention is as follows. [1] In mass %, C: 0.030% or more and 0.500% or less, Si: more than 0.01% and less than 2.50%, Mn: 0.10% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, O: 0.0100% or less and Ti: 0.010% or more and 0.200% or less, Nb: 0.005% or more and 0.500% or less, and the balance being Fe and unavoidable impurities, The microstructure of the steel plate at the 1 / 4 position of the plate thickness is The total area ratio of tempered martensite and bainite is 40% or more and 85% or less, The area ratio of fresh martensite is 0% or more and 25% or less, The area ratio of retained austenite is 5% or more and 20% or less, The remainder is at least one of ferrite and pearlite in an area ratio of 0% to 20%. The Nb is the amount of solid solution Nb (Nb sol) and the Nb content in Nb precipitates with a grain size of less than 20 nm (Nb pre ) and the total amount of Nb (Nb) contained in the steel sheet satisfies the following (Equation 1), A high-strength steel plate having a diffusible hydrogen content of 0.50 mass ppm or less. (Equation 1)(Nb sol / Nb)+(Nb pre / Nb)≧0.40 In (Equation 1), Nb sol :Solute Nb amount (mass%), Nb pre : represents the amount of Nb (mass%) in Nb precipitates with a grain size of less than 20 nm. [2] Furthermore, the component composition is as follows: V: 0.500% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less Co: 1.00% or less, Ni: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, Bi: 0.200% or less, The high-strength steel plate according to [1], containing one or more selected from the following: [3] A high-strength steel sheet according to [1] or [2], having a plating layer on the surface of the steel sheet. [4] The high-strength steel sheet according to [3], wherein the plating layer is an alloyed plating layer. [5] A method for producing a high-strength steel plate according to [1] or [2], The steel material having the above-mentioned composition is heated to a temperature T sola slab heating step of heating at ℃ or higher for 1.0 hour or more; a hot rolling process; Winding temperature T CT a winding step in which the temperature is 650°C or less; Soaking temperature T AT ℃, and the soaking temperature T is 750℃ or more and 950℃ or less. AT and an annealing step of holding the steel sheet at a temperature of 100°C and cooling the steel sheet after the holding step, In the hot rolling process, the finish rolling start temperature T FET °C, finish rolling temperature T FDT °C, effective time t from the start of finish rolling to the end of finish rolling HR Q defined by (Eq. 3) HR and, In the winding step, T FDT ℃ to 650℃ residence time t CT , winding temperature T CT Q defined by (Equation 4) in °C CT and, In the annealing process, the soaking temperature T AT ℃, 650℃ to soaking temperature T AT Heating time t up to °C, soaking temperature T AT Holding time t in °C AT Q defined by (Eq. 5) AT A method for manufacturing high-strength steel plate that satisfies (Equation 6). (Formula 2)T sol =7900 / (3.42-(log([Nb%][C%])))-273 Here, [Nb%] and [C%] are the amount of Nb (mass%) and the amount of C (mass%) contained in the steel, respectively. (Formula 3)Q HR =0.5(T FET +T FDT )×log 10 (t HR ) (Formula 4)Q CT =0.5(T CT +650)×log 10 (t CT ) (Formula 5)Q AT =0.5(650+T AT )×log 10(t) + T AT ×log 10 (t AT ) (Formula 6)Q HR +Q CT +Q AT ≦6000 [6] Q AT The method for producing a high-strength steel plate according to [5], wherein the value of [Strain] is 4700 or less. [7] The method for producing a high-strength steel sheet according to [5] or [6], wherein a plating treatment is carried out after the annealing step. [8] The method for producing a high-strength steel sheet according to [7], wherein the plating treatment is an alloying plating treatment. [Effects of the Invention]

[0019] According to the present invention, a high-strength steel sheet having good formability and excellent LME resistance can be obtained. Therefore, the present invention is highly useful in industrial fields such as automobiles and electrical equipment, and is particularly useful for reducing the weight of automobile body frame parts. DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention will be specifically described below. Note that "%" representing the content of component elements means "% by mass" unless otherwise specified.

[0021] (1) The reason why the composition of the steel in the present invention is limited to the above range will be explained.

[0022] C: 0.030% or more and 0.500% or less C is an element necessary for increasing the strength of tempered martensite, bainite, and fresh martensite. To fully obtain this effect, the C content must be at least 0.030% or more. Therefore, the C content is set to 0.030% or more. Preferably, it is 0.050% or more. More preferably, it is 0.070% or more. Even more preferably, it is 0.090% or more, and most preferably, it is 0.100% or more. On the other hand, if the C content exceeds 0.500%, the weldability and LME resistance properties, which are important when joining automotive parts, will deteriorate. Therefore, the C content is set to 0.500% or less. Preferably, it is 0.400% or less. More preferably, it is 0.300% or less. Even more preferably, it is 0.270% or less, and most preferably, it is 0.250% or less.

[0023] Si: more than 0.01% and less than 2.50% Silicon (Si) is an element that suppresses the excessive formation and growth of carbides in steel, increases the fraction of retained austenite, and improves ductility. If the Si content is 0.01% or less, this effect is reduced and good formability is not achieved. Therefore, the lower limit is set to 0.01%. The Si content is set to more than 0.01%, preferably 0.05% or more, more preferably 0.10% or more, even more preferably 0.50% or more, and most preferably 0.90% or more. However, if the Si content exceeds 2.50%, the melting point of zinc is lowered, facilitating zinc penetration into the steel sheet during welding, resulting in a decrease in the LME resistance of the steel sheet. Therefore, the Si content is set to 2.50% or less, preferably 2.30% or less, more preferably 2.00% or less, even more preferably 1.80% or less, and most preferably 1.60% or less.

[0024] Mn: 0.10% or more and 5.00% or less Mn is an element that affects the area fractions of tempered martensite, bainite, and fresh martensite by improving hardenability. If the Mn content is less than 0.10%, soft phases such as ferrite are excessively formed, making it impossible to obtain the desired area fractions of tempered martensite, bainite, and fresh martensite, resulting in insufficient steel sheet strength. For this reason, the Mn content is set to 0.10% or more. Preferably, it is set to 0.50% or more. More preferably, it is set to 0.80% or more. Even more preferably, it is set to 1.00% or more, and most preferably, it is set to 2.00% or more. On the other hand, if the Mn content exceeds 5.00%, the area fractions of tempered martensite, bainite, and fresh martensite increase, resulting in a decrease in ductility. Therefore, the Mn content is set to 5.00% or less. Preferably, it is set to 4.50% or less. More preferably, it is set to 4.00% or less. Even more preferably, it is set to 3.70% or less, and most preferably, it is set to 3.50% or less.

[0025] P:0.100% or less P may segregate at grain boundaries and cause embrittlement, adversely affecting LME resistance, so its content must be 0.100% or less. Therefore, the P content is set to 0.100% or less, preferably 0.080% or less, more preferably 0.070% or less, even more preferably 0.050% or less, and most preferably 0.040% or less. Although there is no particular lower limit, P is a solid solution strengthening element that can increase the strength of the steel sheet, so it is preferably 0.001% or more, more preferably 0.003% or more, and even more preferably 0.005% or more.

[0026] S: 0.0200% or less S segregates at grain boundaries, embrittling steel during hot working, and may also adversely affect LME resistance through the formation of sulfides, so its content must be 0.0200% or less. Therefore, the S content is set to 0.0200% or less, preferably 0.0180% or less, more preferably 0.0150% or less, even more preferably 0.0100% or less, and most preferably 0.0050% or less. There is no particular lower limit, but due to constraints on production technology, it is preferably set to 0.0001% or more. It is more preferably set to 0.0005% or more, and even more preferably 0.0010% or more.

[0027] Al: 0.100% or less Al acts as a deoxidizer and is an effective element for reducing inclusions in steel, so it is preferable to add it during the deoxidation process. However, Al raises the austenitization transformation point and causes ferrite to be included in the microstructure, so a content of more than 0.100% makes it difficult to achieve the desired TS. Therefore, the Al content is set to 0.100% or less, preferably 0.080% or less, more preferably 0.070% or less, even more preferably 0.060% or less, and most preferably 0.050% or less. Although there is no particular lower limit, the Al content is preferably set to 0.001% or more, more preferably 0.010% or more, and even more preferably 0.020% or more.

[0028] N: 0.0100% or less N has a negative effect on LME resistance by forming coarse nitrides, and if the N content exceeds 0.0100%, a large amount of coarse nitrides is formed, resulting in a significant deterioration of LME resistance. The lower the N content, the better, so the N content is set to 0.0100% or less. Preferably, it is 0.0090% or less. More preferably, it is 0.0080% or less. Even more preferably, it is 0.0070% or less, and most preferably, it is 0.0060% or less. There is no particular lower limit, but due to constraints on production technology, it is preferably 0.0001% or more. It is more preferably 0.0010% or more, and even more preferably, it is 0.0020% or more.

[0029] O: 0.0100% or less O exists as an oxide and reduces the ductility of steel sheet. Therefore, the O content must be 0.0100% or less. Therefore, the O content is set to 0.0100% or less. Preferably, it is 0.0075% or less. More preferably, it is 0.0060% or less. Still more preferably, it is 0.0050% or less, and most preferably, it is 0.0045% or less. There is no particular lower limit for the O content, but due to constraints on production technology, the O content is preferably 0.0001% or more. It is more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0030] Ti: 0.010% or more and 0.200% or less Ti contributes to precipitation strengthening and further refines the prior austenite grain size, which in turn refines tempered martensite and bainite, thereby effectively improving steel strength. To fully achieve these effects, the Ti content is set to 0.010% or more. Preferably, it is 0.012% or more. More preferably, it is 0.015% or more. Even more preferably, it is 0.020% or more. Most preferably, it is 0.025% or more. However, if the Ti content exceeds 0.200%, Ti may remain in an undissolved state during heating of the steel material before hot rolling, increasing the number of coarse precipitates and reducing ductility. Therefore, the Ti content is set to 0.200% or less. Preferably, it is 0.180% or less. More preferably, it is 0.150% or less, even more preferably, it is 0.100% or less, and most preferably, it is 0.050% or less.

[0031] Nb: 0.005% or more and 0.500% or less Nb is an element that improves LME resistance, and the presence of solute Nb in steel during welding has a significant impact on improving LME resistance. Specifically, by dissolving Nb or by controlling the particle radius of precipitated Nb to fall within a certain range, the solute elements present in the steel during welding or the solute elements formed by dissolving the precipitates prevent zinc from penetrating, thereby improving LME resistance. To fully achieve this effect, the Nb content must be 0.005% or more. Preferably, it is 0.007% or more. More preferably, it is 0.008% or more. Even more preferably, it is 0.010% or more, and most preferably, it is 0.012% or more. On the other hand, if the Nb content exceeds 0.500%, coarse precipitates such as Nb carbides and nitrides are formed, which prevents sufficient dissolution of the precipitates during welding, and the improvement in LME resistance due to the solute elements cannot be expected. Furthermore, it is quite conceivable that a lack of El relative to the TS of the steel sheet will cause new cracks to originate from coarse precipitates not only during manufacturing but also after press working and during welding. Therefore, the Nb content is set to 0.500% or less, preferably 0.400% or less, more preferably 0.350% or less, even more preferably 0.200% or less, and most preferably 0.100% or less.

[0032] A high-strength steel sheet according to one embodiment of the present invention has a composition containing the above-mentioned components, with the balance including Fe and unavoidable impurities. Preferably, a high-strength steel sheet according to one embodiment of the present invention has a composition containing the above-mentioned components, with the balance consisting of Fe and unavoidable impurities. Examples of unavoidable impurities include Zn, Pb, and As. A total of 0.100% or less of these impurities is permitted.

[0033] In addition to the above components, the alloy may contain one or more elements selected from the following by mass: V: 0.500% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Co: 1.00% or less, Ni: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less.

[0034] V:0.500% or less V contributes to precipitation strengthening and further refines the prior austenite grain size, thereby refining tempered martensite and bainite, thereby effectively improving steel strength. Therefore, it can be added as needed. While there is no particular lower limit, to achieve the above-mentioned effects, the V content is preferably 0.001% or more. It is more preferably 0.005% or more, and even more preferably 0.010% or more. However, if the V content exceeds 0.500%, V may remain in an undissolved state during heating of the steel material before hot rolling, increasing the number of coarse precipitates and reducing ductility. Therefore, when V is contained, the V content is set to 0.500% or less. It is preferably set to 0.400% or less. It is more preferably set to 0.300% or less, even more preferably set to 0.200% or less, and most preferably set to 0.100% or less.

[0035] Ta: 0.10% or less Like Ti, Ta contributes to high strength by forming alloy carbides and alloy carbonitrides. Additionally, Ta can be added as needed to partially dissolve in Nb carbides and Nb carbonitrides to form composite precipitates such as (Nb, Ta)(C, N), significantly suppressing precipitate coarsening and stabilizing the contribution of precipitation strengthening to strength. While there is no particular lower limit, to achieve the above-mentioned effects, the Ta content is preferably 0.01% or more. It is more preferably 0.02% or more, and even more preferably 0.03% or more. However, excessive Ta content saturates the precipitate stabilization effect and increases alloy costs. Therefore, when Ta is added, the Ta content is set to 0.10% or less, preferably 0.08% or less, more preferably 0.07% or less, even more preferably 0.06% or less, and most preferably 0.05% or less.

[0036] W: 0.10% or less W can be added as needed to improve the hardenability of steel and further improve steel strength by refining tempered martensite and bainite. While there is no particular lower limit, to achieve the above-mentioned effects, the W content is preferably 0.01% or more. It is more preferably 0.02% or more, and even more preferably 0.03% or more. However, if the W content exceeds 0.10%, the amount of coarse precipitates such as WN and WS remaining in an undissolved state during slab heating in hot rolling may increase, resulting in reduced ductility. Therefore, when W is added, the W content is set to 0.10% or less. It is preferably set to 0.08% or less. It is more preferably set to 0.07% or less. It is even more preferably set to 0.06% or less, and most preferably set to 0.05% or less.

[0037] B: 0.0100% or less B is an element that can improve hardenability by segregating at austenite grain boundaries. It forms a structure mainly composed of tempered martensite and bainite, which can improve the strength of the steel sheet. It also contributes to improving LME resistance, so it can be added as needed. While there is no particular lower limit, to achieve the above-mentioned effects, the B content is preferably 0.0003% or more. It is more preferably 0.0005% or more, and even more preferably 0.0007% or more. However, if the B content exceeds 0.0100%, coarse precipitates are formed, resulting in a decrease in ductility. Therefore, when B is added, the B content is set to 0.0090% or less. It is preferably set to 0.0080% or less. It is more preferably set to 0.0070% or less. It is even more preferably set to 0.0050% or less, and most preferably set to 0.0030% or less.

[0038] Cr:1.00% or less Cr has the effect of improving the balance between strength and ductility, so it can be added as needed. Although there is no particular lower limit, to obtain the above effect, the Cr content is preferably 0.01% or more. 0.05% or more is more preferable, and 0.07% or more is even more preferable. However, if it is added in excess of 1.00%, the area ratio of fresh martensite becomes excessive, and dimensional accuracy and ductility during forming decrease. Therefore, when Cr is added, the Cr content is set to 1.00% or less. Preferably, it is set to 0.80% or less. More preferably, it is set to 0.60% or less. Even more preferably, it is set to 0.50% or less. Most preferably, it is set to 0.30% or less.

[0039] Mo: 1.00% or less Mo has the effect of improving the balance between strength and ductility, and can be added as needed. While there is no particular lower limit, to obtain the above-mentioned effects, the Mo content is preferably 0.01% or more. It is more preferably 0.05% or more, and even more preferably 0.07% or more. However, if Mo is added in excess of 1.00%, the area ratio of fresh martensite becomes excessive, resulting in reduced dimensional accuracy and ductility during forming. Therefore, when Mo is added, the Mo content is set to 1.00% or less. It is preferably set to 0.80% or less. It is more preferably set to 0.50% or less. It is even more preferably set to 0.30% or less, and most preferably set to 0.20% or less.

[0040] Co: 1.00% or less Co is an element effective in improving hardenability and strengthening steel, and can be added as needed. While there is no particular lower limit, to achieve the above-mentioned effects, the Co content is preferably 0.01% or more. It is more preferably 0.05% or more, and even more preferably 0.07% or more. However, if the Co content exceeds 1.00%, the area fraction of fresh martensite becomes excessively large, resulting in reduced dimensional accuracy and ductility during forming. Therefore, when Co is added, the Co content is set to 1.00% or less. It is preferably set to 0.80% or less. It is more preferably set to 0.60% or less. It is even more preferably set to 0.30% or less, and most preferably set to 0.20% or less.

[0041] Ni: 1.00% or less Ni increases the strength of steel through solid solution strengthening, so it can be added as needed. While there is no particular lower limit, to obtain the above-mentioned effects, the Ni content is preferably 0.01% or more. It is more preferably 0.05% or more, and even more preferably 0.07% or more. However, if Ni is added in excess of 1.00%, the area ratio of fresh martensite becomes excessive, resulting in reduced dimensional accuracy and ductility during forming. Therefore, when Ni is added, the Ni content is set to 1.00% or less. It is preferably set to 0.80% or less. It is more preferably set to 0.60% or less. It is even more preferably set to 0.30% or less, and most preferably set to 0.20% or less.

[0042] Cu:1.00% or less Cu is an element effective in strengthening steel and can be added as needed. While there is no particular lower limit, to achieve the above-mentioned effects, the Cu content is preferably 0.01% or more. A Cu content of 0.05% or more is more preferable, and a Cu content of 0.07% or more is even more preferable. However, if the Cu content exceeds 1.00%, the area ratio of tempered martensite, bainite, and fresh martensite becomes excessive, resulting in reduced dimensional accuracy and ductility during forming. Therefore, when Cu is added, the Cu content is set to 1.00% or less. It is preferably set to 0.80% or less. It is more preferably set to 0.60% or less. It is even more preferably set to 0.30% or less, and most preferably set to 0.20% or less.

[0043] Sn: 0.200% or less, Sb: 0.200% or less Sn and Sb suppress decarburization in a region of several tens of micrometers in the surface layer of the steel sheet, which occurs due to nitriding or oxidation of the steel sheet surface, and prevent a decrease in the area ratio of tempered martensite on the steel sheet surface. In addition to this effect, Sn and Sb can be added as needed to ensure strength and material stability. While there is no particular lower limit, to achieve the above effects, their contents are preferably 0.001% or more. 0.003% or more is more preferable, and 0.005% or more is even more preferable. However, excessive content of any of these elements exceeding 0.200% may cause the steel sheet to become embrittled, resulting in a decrease in ductility. Therefore, when Sn and Sb are contained, their contents are set to 0.200% or less, preferably 0.100% or less, more preferably 0.070% or less, even more preferably 0.050% or less, and most preferably 0.030% or less.

[0044] Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less If the Ca, Mg, and REM contents are each 0.0100% or less, coarse precipitates and inclusions do not increase, and Nb precipitation is not affected, so LME resistance does not deteriorate. Therefore, when Ca, Mg, and REM are contained, the Ca, Mg, and REM contents are set to 0.0100% or less. Preferably, they are set to 0.0050% or less. More preferably, they are set to 0.0040% or less, even more preferably, 0.0035% or less, and most preferably, 0.0030% or less. Although there are no particular lower limits for the Ca, Mg, and REM contents, because these elements spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet, the Ca, Mg, and REM contents are preferably set to 0.0001% or more. More preferably, they are set to 0.0005% or more. Even more preferably, they are set to 0.0007% or more, and most preferably, they are set to 0.0010% or more.

[0045] Zr: 0.100% or less, Te: 0.100% or less If Zr and Te are each 0.100% or less, coarse precipitates and inclusions do not increase, and they do not affect Nb precipitation, so LME resistance does not deteriorate. Therefore, when Zr and Te are contained, the Zr and Te contents are set to 0.100% or less. Preferably, they are set to 0.080% or less. More preferably, they are set to 0.070% or less, even more preferably, 0.060% or less, and most preferably, 0.050% or less. Although there are no particular lower limits for the Zr and Te contents, since these elements spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet, the Zr and Te contents are preferably set to 0.001% or more. More preferably, they are set to 0.010% or more, and even more preferably, 0.020% or more.

[0046] Hf: 0.10% or less If Hf is 0.10% or less, coarse precipitates and inclusions do not increase, and Hf does not affect the precipitation of Nb, so LME resistance does not deteriorate. Therefore, when Hf is contained, the Hf content is set to 0.10% or less. Preferably, it is set to 0.080% or less. More preferably, it is set to 0.070% or less, even more preferably, it is set to 0.060% or less, and most preferably, it is set to 0.050% or less. Although there is no particular lower limit for the Hf content, since Hf is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformability of the steel sheet, when Hf is contained, the Hf content is preferably set to 0.003% or more. More preferably, it is set to 0.010% or more. More preferably, it is set to 0.020% or more, and even more preferably, it is set to 0.030% or more.

[0047] Bi:0.200% or less If Bi is 0.200% or less, coarse precipitates and inclusions do not increase, and Bi does not affect the precipitation of Nb, so LME resistance does not deteriorate. Therefore, when Bi is contained, the Bi content is set to 0.200% or less. Preferably, it is 0.100% or less. More preferably, it is 0.050% or less, even more preferably, it is 0.030% or less, and most preferably, it is 0.020% or less. There is no particular lower limit for the Bi content, but because Bi is an element that reduces segregation, the Bi content is more preferably 0.001% or more. More preferably, it is 0.005% or more, and even more preferably, it is 0.010% or more.

[0048] In addition, when the content of each of the above-mentioned V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf, and Bi is less than the preferable lower limit, the effect of the present invention is not impaired, and therefore these elements are included as unavoidable impurities.

[0049] (2) Next, we will explain the microstructure.

[0050] Area ratio of tempered martensite and bainite: 40% to 85% Tempered martensite and bainite contribute to the strength of steel sheets. Specifically, having a steel sheet mainly comprised of tempered martensite and bainite is effective in maintaining high strength. To fully achieve this effect, the total area fraction of bainite and tempered martensite must be at least 40% or more. Preferably, it is 42% or more. More preferably, it is 45% or more, even more preferably, it is 47% or more, and most preferably, it is 50% or more. On the other hand, if the sum of the area fractions of bainite and tempered martensite exceeds 85%, the steel structure will be dominated by hard phases such as tempered martensite, bainite, and fresh martensite. Furthermore, since these hard phases constrain the retained austenite, the TRIP effect will not be effectively manifested, and ductility will be poor, making it difficult to achieve good formability. Therefore, the total area fraction of bainite and tempered martensite must be 85% or less. Preferably, it is 83% or less. More preferably, it is 82% or less, even more preferably, it is 81% or less, and most preferably, it is 80% or less.

[0051] Area ratio of fresh martensite: 0% to 25% Fresh martensite is a very hard phase, which improves the strength of steel. While fresh martensite is not necessarily required if the strength of the steel sheet is ensured, the inclusion of fresh martensite in the steel sheet structure further improves the strength of the steel sheet, making it possible to achieve even higher strength. Therefore, the area fraction of fresh martensite must be 0% or more. It is preferably 2% or more. More preferably, it is 3% or more. Even more preferably, it is 4% or more, and most preferably, it is 5% or more. On the other hand, fresh martensite reduces the ductility of steel, making it difficult to achieve good formability. Therefore, the area fraction of fresh martensite must be 25% or less. It is preferably 23% or less. More preferably, it is 22% or less. Even more preferably, it is 21% or less, and most preferably, it is 20% or less.

[0052] Area ratio of retained austenite: 5% to 20% During processing, retained austenite transforms into martensite due to the TRIP effect, increasing strength and improving ductility by enhancing strain dispersion. Therefore, to ensure good formability, the area fraction of retained austenite must be 5% or more. It is preferably 7% or more, more preferably 8% or more, even more preferably 9% or more, and most preferably 10% or more. On the other hand, if the area fraction of retained austenite exceeds 20%, elements concentrated in the retained austenite, particularly Si, diffuse during welding, lowering the melting point of zinc. This facilitates zinc penetration into the steel sheet during welding, potentially deteriorating the LME resistance of the steel sheet. Therefore, to ensure sufficient ductility while preventing deterioration of LME resistance, the area fraction of retained austenite must be 20% or less. It is preferably 18% or less, more preferably 17% or less, even more preferably 16% or less, and most preferably 15% or less.

[0053] In the present invention, even if a residual structure such as ferrite or pearlite is present, the effects of the present invention are not impaired. When a residual structure is present, the residual structure includes at least one of ferrite and pearlite, and specifically, the area ratio of at least one of ferrite and pearlite is 0% or more. The area ratio of the residual structure is preferably 20% or less, more preferably 18% or less, even more preferably 15% or less, and most preferably 13% or less.

[0054] The amount of dissolved Nb in the steel sheet (Nb sol ) and the Nb content in Nb precipitates with a grain size of less than 20 nm (Nb pre ) and the total amount of Nb (Nb) contained in the steel sheet satisfies the following (Equation 1): (Equation 1)(Nb sol / Nb)+(Nb pre / Nb)≧0.40 In (Equation 1), Nb sol :Solute Nb amount (mass%), Nb pre : represents the amount of Nb (mass%) in Nb precipitates with a grain size of less than 20 nm. The amount of dissolved Nb in the steel sheet (Nb sol , unit is mass %, and the Nb content in Nb precipitates with a grain size of less than 20 nm (Nb pre , in mass %, and the total amount of Nb (Nb) contained in the steel is one of the important constituents of the present invention. In the present invention, LME due to transfer LME is prevented and LME resistance is improved by making the Nb state in the steel sheet solid solution or making it possible to make it solid solution during welding. The inventors varied the state of Nb contained in the steel sheet by producing various steels, and found that LME resistance is improved when (Equation 1) is satisfied. The mechanism by which Nb in the steel in a solid solution state during welding improves LME resistance is not clear, but it is thought that when Nb exists in a solid solution state, it prevents zinc from penetrating into the steel sheet, which is the direct cause of LME cracking. From the above, it is necessary to satisfy (Equation 1). (Nb sol / Nb)+(Nb pre / Nb) is preferably 0.41 or more, and more preferably 0.42 or more. There is no particular upper limit to the value, but (Nb sol / Nb)+(Nb pre / Nb) is preferably 0.95 or less, and more preferably 0.90 or less. The lower limit of the Nb precipitates is not particularly limited, but the particle size may be 0.1 nm or more.

[0055] Diffusible hydrogen content in steel is 0.50 mass ppm or less In order to ensure ductility and have good formability, the amount of diffusible hydrogen in the steel is set to 0.50 ppm by mass or less, preferably 0.30 ppm by mass or less, more preferably 0.25 ppm by mass or less, even more preferably 0.20 ppm by mass or less, and most preferably 0.15 ppm by mass or less. There is no particular lower limit for the amount of diffusible hydrogen in the steel, but due to constraints on production technology, the amount of diffusible hydrogen in the steel is preferably 0.01 ppm by mass or more, more preferably 0.02 ppm by mass or more, even more preferably 0.03 ppm by mass or more, and most preferably 0.05 ppm by mass or more.

[0056] (3) Next, the manufacturing method and manufacturing conditions will be explained.

[0057] [Slab (steel material) heating process] Heating conditions for steel material: Temperature T expressed by (Equation 2) sol Heat at ℃ or higher for 1.0 hours or more Precipitates that exist during the heating stage of steel material remain as coarse precipitates in the final steel sheet, which not only adversely affect the LME resistance characteristics but also do not contribute to strength and cannot be expected to improve the TS × El balance. Therefore, it is necessary to re-melt as many coarse precipitates that precipitated during casting as possible. sol If the heating temperature is below T °C or the heating time is less than 1 hour, the solid solution of Nb becomes insufficient and formula 1 is not satisfied, which leads to poor LME resistance and an increased risk of problems occurring during hot rolling due to an increased rolling load. Also, from the perspective of scaling off defects such as bubbles and segregations in the surface layer of the steel material, reducing cracks and irregularities on the steel sheet surface, and achieving a smooth steel sheet surface, the heating temperature of the steel material should be T sol °C or higher. Therefore, the heating temperature of the steel material is T sol °C or higher. Preferably, T sol × 1.1°C or more. More preferably, T sol × 1.2°C or more. More preferably, T sol × 1.3 ° C or more, and most preferably T sol× 1.5°C or higher. Furthermore, the heating time at the above temperature is 1.0 hour or longer, preferably 1.1 hours or longer, more preferably 1.2 hours or longer, even more preferably 1.3 hours or longer, and most preferably 1.5 hours or longer. Furthermore, there is no particular upper limit to the heating temperature of the steel material, but if it exceeds 1500°C, the amount of oxidation increases and scale loss increases, so the heating temperature of the steel material is preferably 1500°C or lower, more preferably 1450°C or lower, and even more preferably 1400°C or lower. (Formula 2)T sol =7900 / (3.42-(log([Nb%][C%])))-273

[0058] [Hot rolling process] Finishing rolling start temperature for hot rolling: T FET °C The heated steel material is hot-rolled to produce a hot-rolled steel sheet. At this time, the upper and lower limits of the start temperature of the finish rolling are not particularly defined. sol At temperatures below 100°C, the Nb dissolved during heating of the steel material precipitates, the precipitate grain size becomes large, and the formula (1) is no longer satisfied, resulting in poor LME resistance. Therefore, the finish rolling start temperature is set to T sol It is preferable to set the temperature at 100°C or higher. sol ℃-70℃ or more. More preferably, T sol °C - 50 °C or higher. Most preferably T sol ℃-30℃ or higher. On the other hand, if the finish rolling start temperature exceeds 1200℃, scale loss during slab preheating increases and it may cause the sheet to crack during hot rolling. For this reason, the finish rolling start temperature in hot rolling is preferably 1200℃ or lower, more preferably 1170℃ or lower, even more preferably 1150℃ or lower, and most preferably 1120℃ or lower.

[0059] Finishing temperature of hot rolling: T FDT °C The heated steel material is hot-rolled to produce a hot-rolled steel sheet. While there are no upper or lower limits for the finish temperature of finish rolling, if the finish temperature is below 800°C, Nb dissolved during heating of the steel material precipitates, resulting in an increased amount of Nb precipitation. Furthermore, the rolling load increases, which may hinder cold rolling. Therefore, the finish temperature of hot rolling is preferably 800°C or higher, more preferably 820°C or higher, even more preferably 840°C or higher, and most preferably 850°C or higher. Furthermore, if the finish temperature exceeds 1000°C, the amount of oxide (scale) generated increases sharply, the interface between the base steel and the oxide becomes rough, and the surface quality after pickling and cold rolling tends to deteriorate. Furthermore, the crystal grains become excessively coarse, which may cause surface roughness in pressed products during processing. Therefore, the finish temperature is preferably 1000°C or lower, more preferably 980°C or lower, and even more preferably 970°C or lower. The most preferable temperature is 950°C or lower.

[0060] Effective time: t HR The time required from the start of finish rolling to the end of finish rolling is called effective time t HR Since Nb contained in the steel sheet begins to precipitate and grow during finish rolling, the effective time t HR is one of the parameters for controlling the morphology of Nb to improve LME resistance. HR Although there is no particular upper or lower limit for the effective time t, it is preferably 3 seconds or more, more preferably 4 seconds or more, even more preferably 5 seconds or more, and most preferably 7 seconds or more. HR is preferably 15 seconds or less, more preferably 12 seconds or less, even more preferably 11 seconds or less, and most preferably 10 seconds or less.

[0061] [Winding process] T FDT Residence time from ℃ to 650℃: t CT The time from the finish rolling temperature to 650°C is called the residence time t CTFrom the time when finish rolling is completed until the sheet temperature reaches 650°C, the precipitation of Nb and the growth of precipitated Nb are likely to proceed. CT is one of the parameters for controlling the morphology of Nb to improve LME resistance. CT Although there is no particular upper or lower limit for the residence time t, it is preferably 5 seconds or more, more preferably 7 seconds or more, even more preferably 8 seconds or more, and most preferably 9 seconds or more. CT The time is 20 seconds or less, more preferably 18 seconds or less, even more preferably 17 seconds or less, and most preferably 15 seconds or less.

[0062] Coiling temperature after hot rolling T CT ℃: 650℃ or less If the coiling temperature after hot rolling exceeds 650°C, the precipitation of Nb and the growth of the precipitated Nb will proceed excessively, and Equation 1 will no longer be satisfied, which may cause deterioration of LME resistance. In addition, an oxide film that is difficult to remove by pickling will form on the surface of the hot-rolled sheet, which may cause a deterioration in the surface appearance after cold rolling. Therefore, the coiling temperature after hot rolling is set to 650°C or lower. Preferably, it is set to 630°C or lower. More preferably, it is set to 620°C or lower. Still more preferably, it is set to 610°C or lower, and most preferably, it is set to 600°C or lower. Note that there is no particular lower limit for the coiling temperature. However, if the coiling temperature is lower than 300°C, the strength of the hot-rolled sheet will increase, the rolling load in cold rolling will increase, and defects in the sheet shape will occur, resulting in a decrease in productivity. Therefore, the lower limit of the coiling temperature is preferably set to 300°C or higher. More preferably, it is set to 320°C or higher. Even more preferably, it is set to 350°C or higher. Most preferably, it is set to 400°C or higher.

[0063] The obtained hot-rolled steel sheet (hot-rolled coil) may be subjected to intermediate heat treatment at a temperature of less than 650°C as necessary to prevent an increase in load during subsequent cold rolling. In this case, the temperature is preferably 150°C or higher. For example, heat treatment may be performed in a box annealing furnace with a soaking temperature of 500°C and a soaking time of 4 hours.

[0064] The obtained hot-rolled steel sheet (hot-rolled coil) may be subjected to treatment such as pickling, if necessary. The pickling method for the hot-rolled coil may be a conventional method. Furthermore, the hot-rolled coil may be subjected to skin-pass rolling in order to correct the shape and improve the pickling properties.

[0065] After hot rolling and / or intermediate heat treatment and / or pickling, the steel may be subjected to an annealing step (heat treatment) as described below, or may be subjected to cold rolling followed by heat treatment. When cold rolling is performed, the cold reduction is preferably 25% or more, more preferably 30% or more, even more preferably 32% or more, and most preferably 35% or more. On the other hand, excessive reduction increases the rolling load and leads to an increase in the load on the cold rolling mill, so the upper limit of the cold rolling reduction is preferably 75% or less, more preferably 70% or less, even more preferably 67% or less, and most preferably 65% ​​or less.

[0066] [Annealing process] 650℃ to soaking temperature T AT Heating time to °C: t When the sheet temperature reaches 650°C or higher, precipitation of Nb and growth of the precipitated Nb progress, so the temperature rise time t is one of the parameters for controlling the morphology of Nb to improve LME resistance. While there are no upper or lower limits set for the temperature rise time t, it is preferably 300 seconds or more. More preferably, it is 400 seconds or more. It is even more preferably 450 seconds or more, and most preferably 490 seconds or more. Furthermore, the temperature rise time t is preferably 700 seconds or less. More preferably, it is 650 seconds or less. It is even more preferably 600 seconds or less, and most preferably 590 seconds or less.

[0067] Soaking temperature: T AT is between 750℃ and 950℃ If the temperature is maintained below 750°C, the steel sheet will be held in the two-phase region, and the area ratio of tempered martensite and bainite in the final structure will be less than 40%, making it impossible to ensure sufficient steel sheet strength. AT°C is set to 750 °C or higher, preferably 770 °C or higher, more preferably 800 °C or higher, and even more preferably 840 °C or higher. On the other hand, if the temperature is maintained in a range above 950 °C, Nb tends to precipitate, and the growth of Nb precipitates proceeds excessively, so that (Equation 1) is no longer satisfied, and the LME resistance property deteriorates. For this reason, the soaking temperature T AT °C is 950°C or less, preferably 940°C or less, more preferably 930°C or less, and further preferably 920°C or less.

[0068] Soaking temperature T AT Holding time in °C: t AT Soaking temperature T AT Holding time t in °C AT is one of the control parameters for promoting the austenitization of steel sheets during soaking, and is also one of the parameters for controlling LME resistance, since it is also related to the precipitation and growth of Nb precipitates. AT Although there are no particular upper or lower limits, in order to sufficiently promote austenitization, it is preferably 15 seconds or more, more preferably 30 seconds or more, even more preferably 50 seconds or more, and most preferably 100 seconds or more. Furthermore, from the viewpoint of making the grains of Nb precipitates fine and improving LME resistance, it is preferably 1000 seconds or less, more preferably 500 seconds or less, even more preferably 400 seconds or less, and most preferably 350 seconds or less.

[0069] Q HR and Q CT and Q AT The total is 6000 or less In order to suppress the precipitation of Nb and the growth of Nb precipitates during heating in the hot rolling, coiling, and annealing processes, the finish rolling start temperature T FET °C, finish rolling temperature T FDT °C, effective time t from the start of finish rolling to the end of finish rolling HR Q defined by the following (Equation 3) HR In the winding process, T FDT Residence time t from 650℃CT , winding temperature T CT Q defined by the following formula (4) in °C CT In the annealing process, the soaking temperature T AT ℃, 650℃ to soaking temperature T AT Heating time t up to °C, soaking temperature T AT Holding time t in °C AT Q defined by the following (Equation 5) AT satisfies the following (equation 6), that is, Q HR and Q CT and Q AT It is an important requirement in the present invention that the sum of these is 6000 or less, and the reason for this will be explained below. (Formula 3)Q HR =0.5(T FET +T FDT )×log 10 (t HR ) (Formula 4)Q CT =0.5(T CT +650)×log 10 (t CT ) (Formula 5)Q AT =0.5(650+T AT )×log 10 (t) + T AT ×log 10 (t AT ) (Formula 6)Q HR +Q CT +Q AT ≦6000 In order to suppress the precipitation and growth of Nb during the hot rolling, coiling and heating processes, T sol The effective time t when heat of 650°C or more is applied is HR , residence time t CT , heating time t, holding time t AT For each of the Q HR and Q CT and Q AT It is an important requirement in the present invention that the sum of these is 6000 or less.

[0070] Q HR and Q CT and QAT If the sum of Q exceeds 6000, Nb will precipitate excessively and the precipitates will grow, reducing the amount of Nb that improves LME resistance, resulting in poor LME resistance. HR and Q CT and Q AT The sum of these is 6000 or less, preferably 5990 or less, more preferably 5980 or less, even more preferably 5970 or less, and most preferably 5950 or less. HR and Q CT and Q AT Although there is no particular lower limit for the sum of these, in consideration of the range that can be achieved in actual operation, it is preferably 3500 or more, more preferably 3600 or more, even more preferably 3700 or more, most preferably 3800 or more, and most preferably 3900 or more.

[0071] Q AT Value is 4700 or less (optimal condition) By taking into consideration the heat applied during the hot rolling, coiling, and annealing processes, Q HR and Q CT and Q AT Therefore, the upper and lower limits of each value are not specifically set, but from the viewpoint of suppressing excessive growth of Nb during heating in the annealing process, which is directly related to the final structure of the steel sheet, Q AT It is preferable that Q be 4700 or less, more preferably 4650 or less, and even more preferably 4630 or less. It is most preferably 4600 or less. From the viewpoint of increasing the strength of steel, in order to more precisely control the austenite fraction during annealing, Q AT is preferably 3500 or more, more preferably 3800 or more, even more preferably 4100 or more, and most preferably 4120 or more.

[0072] Similarly, in the hot rolling process and coiling process, from the viewpoint of suppressing excessive growth of Nb, Q HRIt is preferable that Q is 1200 or less, more preferably 1170 or less, and further preferably 1150 or less. CT It is preferable that Q is 850 or less, more preferably 840 or less, and further preferably 835 or less. On the other hand, there is no particular lower limit, but Q HR It is preferable to set the Q value to 250 or more. HR is more preferably 600 or more, and even more preferably 700 or more. CT It is preferable to set the Q factor to 150 or more. CT is more preferably 170 or more, and even more preferably 180 or more.

[0073] In the annealing process, the steel sheet is heated to a soaking temperature T AT and retention time t AT After holding the temperature for a certain period, the material may be cooled as is, or may be cooled to an arbitrary holding temperature and then held, or may be cooled to an arbitrary cooling stop temperature, then reheated to the holding temperature, held, and then cooled.

[0074] Furthermore, reheating may be performed after the annealing process. Forming martensite in the steel sheet and then tempering it concentrates C in the untransformed austenite, improving the stability of the austenite and thereby increasing the area fraction of retained austenite contained in the steel sheet after cooling, thereby further improving ductility. For this reason, when reheating is performed, it is preferable to cool the steel sheet to a cooling stop temperature of 100°C or higher and 350°C or lower, reheat it to a holding temperature of 200°C or higher and 450°C or lower, hold it, and then cool it to room temperature. The reheating temperature is preferably 200°C or higher, more preferably 210°C or higher, even more preferably 230°C or higher, and most preferably 250°C or higher. The reheating temperature is preferably 450°C or lower, more preferably 430°C or lower, even more preferably 410°C or lower, and most preferably 400°C or lower.

[0075] Furthermore, if reheating is not performed, it is preferable to hold the temperature at 200°C or higher and 450°C or lower for 10 seconds or longer and then cool to room temperature. It is more preferable to hold the temperature at 200°C or higher and 450°C or lower for 20 seconds or longer, and even more preferable to hold it for 25 seconds or longer. There is no particular upper limit to the holding time, but it is preferably 500 seconds or shorter, and more preferably 400 seconds or shorter.

[0076] Plating For example, when hot-dip galvanizing is performed, the annealed steel sheet is immersed in a galvanizing bath at 440°C or higher and 500°C or lower to perform the hot-dip galvanizing treatment, and then the coating weight is adjusted by gas wiping or the like. The temperature of the galvanizing bath is preferably 440°C or higher, more preferably 450°C or higher, and even more preferably 455°C or higher. The temperature of the galvanizing bath is also preferably 500°C or lower, more preferably 490°C or lower, and even more preferably 485°C or lower. There are no special restrictions on the plating conditions, but the plating weight (amount of plating per side) is set at 20 g / m from the viewpoint of corrosion resistance and plating weight control. 2 The coating weight is preferably 25 g / m or more. 2 More preferably, it is 30 g / m or more. 2 It is even more preferable that the thickness is 32 g / m or more. 2 From the viewpoint of adhesion, it is most preferable to set the thickness to 120 g / m or more. 2 The coating weight is preferably 100 g / m or less. 2 It is more preferable that the density is 70 g / m or less. 2 It is even more preferable that the thickness is 65 g / m or less. 2 Most preferably, the following is true:

[0077] It is preferable that the hot-dip galvanizing be performed using a galvanizing bath containing 0.08% or more and 0.30% or less of Al. That is, the Al content in the hot-dip galvanizing is preferably 0.08% or more, more preferably 0.09% or more, even more preferably 0.10% or more, and most preferably 0.12% or more. The Al content in the hot-dip galvanizing is preferably 0.30% or less, more preferably 0.25% or less, even more preferably 0.22% or less, and most preferably 0.20% or less. Furthermore, the effects of the present invention remain unchanged even if the plating bath contains elements other than Al, Mg, and Si, such as Pb, Sb, Fe, Mg, Mn, Ni, Ca, Ti, V, Cr, Co, and Sn.

[0078] When hot-dip galvanizing is performed, the hot-dip galvanizing treatment is performed in a temperature range of 450°C to 600°C after the hot-dip galvanizing treatment. If the hot-dip galvanizing treatment is performed at a temperature above 600°C, untransformed austenite may transform into pearlite, resulting in an area ratio of retained austenite of less than 5%, and ductility may be reduced. Therefore, when hot-dip galvanizing treatment is performed, the hot-dip galvanizing treatment is preferably performed in a temperature range of 450°C or higher. 460°C or higher is more preferable. 465°C or higher is even more preferable, and 470°C or higher is most preferable. When hot-dip galvanizing treatment is performed, the hot-dip galvanizing treatment is preferably performed in a temperature range of 600°C or lower. 570°C or lower is more preferable. 550°C or lower is even more preferable, and 530°C or lower is most preferable. The Fe concentration in the coating layer of the alloyed hot-dip galvanized steel sheet is preferably 8 to 17%. That is, the Fe concentration in the coating layer of the alloyed hot-dip galvanized steel sheet is preferably 8% or more, more preferably 9% or more, and even more preferably 10% or more. The Fe concentration in the coating layer of the alloyed hot-dip galvanized steel sheet is preferably 17% or less, more preferably 16% or less, and even more preferably 15% or less. The alloyed plating layer is formed by performing alloying plating treatment. [Example]

[0079] Steels having the chemical compositions shown in Tables 1-1 and 1-2, with the balance being Fe and unavoidable impurities, were melted in a converter and continuously cast into slabs. The resulting slabs underwent the heating, hot-rolling, coiling, and annealing processes under the conditions shown in Tables 2-1 and 2-2. If reheating was performed, the slabs were cooled to a cooling stop temperature of 100°C to 350°C, reheated to a holding temperature of 200°C to 450°C, and then cooled to room temperature. If reheating was not performed, the slabs were held at a holding temperature of 200°C to 450°C for 10 seconds or more, cooled to room temperature, and cold-rolled at a rolling reduction of 50% to obtain high-strength cold-rolled steel sheets (CR). Some steel sheets were subjected to the coiling process and then to the heating process without cold rolling to obtain hot-rolled steel sheets (HR). Furthermore, some steel sheets were subjected to hot-dip galvanizing to obtain hot-dip galvanized steel sheets (GI) and galvannealed steel sheets (GA). The hot-dip galvanizing bath used for the galvanized steel sheet (GI) contained 0.19% Al by mass, and for the galvannealed steel sheet (GA) contained 0.14% Al by mass. The bath temperature was 465°C. The coating weight per side was 45 g / m 2 The GA was adjusted so that the Fe concentration in the plating layer was within the range of 9 mass % or more and 12 mass % or less.

[0080] The cross-sectional microstructure, tensile properties, and LME resistance properties of the obtained steel sheets were investigated, and the results are shown in Tables 3-1 and 3-2.

[0081] [Table 1-1]

[0082] [Table 1-2]

[0083] [Table 2-1]

[0084] [Table 2-2]

[0085] [Table 3-1]

[0086] [Table 3-2]

[0087] The area fractions of fresh martensite, tempered martensite, and bainite were determined by polishing a thickness cross section (L cross section) parallel to the rolling direction of the steel plate, etching it with nital, and observing 10 fields of view at 2000x magnification using a scanning electron microscope (SEM) at a position 1 / 4 of the plate thickness (a position corresponding to 1 / 4 of the plate thickness in the depth direction from the steel plate surface).The area fractions of each structure (the total of tempered martensite and bainite, and fresh martensite) were calculated using the obtained structural images.In addition, in the above structural images, fresh martensite was defined as the light gray structure region, and tempered martensite and bainite were defined as the dark gray structure region where carbides precipitated.

[0088] The area fraction of retained austenite was determined by polishing the steel plate from the 1 / 4 position to a depth of 0.1 mm, then chemically polishing the surface for a further 0.1 mm. The surface was then measured using CoKα radiation in an X-ray diffractometer to measure the integrated intensity ratios of the diffraction peaks for the {200}, {220}, and {311} planes of fcc iron and the {200}, {211}, and {220} planes of bcc iron, and then averaging the nine integrated intensity ratios obtained.

[0089] To measure the amount of hydrogen in steel, hot-rolled, cold-rolled, or galvanized steel sheets were prepared as test pieces measuring approximately 5 x 30 mm. For galvanized steel sheets, the plating on the surface was removed using a router (precision grinder) before the sheets were placed in a quartz tube. The atmosphere inside the tube was replaced with Ar, and the temperature was raised at 200°C / hr. The amount of hydrogen released up to 400°C was measured using gas chromatography by temperature-programmed analysis. The cumulative amount of hydrogen detected in the temperature range from room temperature (25°C) to less than 250°C was taken as the amount of diffusible hydrogen.

[0090] All Nb content in steel (Nb total ), the amount of dissolved Nb in the steel sheet (Nb sol ) and the amount of Nb with a particle size of less than 20 nm (Nb pre) was measured using the following procedure. First, the total amount of Nb (Nb) contained in the steel was measured by wet chemical analysis. Next, the method for measuring the amount of Nb with a particle size of 20 nm or less will be explained. For the amount of Nb precipitates of 20 nm or less, precipitates in the steel were captured as residuals, the amount of Nb in the total residual was determined, and then the amount of Nb present in the residuals with a particle size of 20 nm or more was determined, and the amount of Nb was calculated by taking the difference between the amount of Nb in the total residual and the amount of Nb present in the residuals with a particle size of 20 nm or more. The specific procedure is as follows: Multiple test pieces of hot-rolled steel sheet, cold-rolled steel sheet, or galvanized steel sheet cut to approximately 20 x 50 mm were prepared, and for galvanized steel sheet, the plating on the surface of the test piece was removed using a router (precision grinder). The surfaces of the sampled test pieces were pre-polished to a depth of approximately 50 μm to obtain a new surface. The resulting specimens were electrolyzed using 10 vol% acetylacetone-1 mass% tetramethylammonium chloride-methanol as an electrolyte for deposit extraction. The resulting electrolytic solution was passed through a 0.2 μm pore size filter to capture the residue. The residue was then acid-decomposed and the Nb concentration was quantified in mass% using ICP atomic emission spectrometry. This was used as the Nb content in the total residue. The remaining specimens were then electrolyzed using the same 10 vol% acetylacetone-1 mass% tetramethylammonium chloride-methanol solution. The residues remaining on the electrolytic metal specimens were immersed in separately prepared methanol and collected in a container using ultrasonic vibration. The electrolytic solution and the methanol containing the residues remaining on the metal specimens were then collected using an alumina filter with a 20 nm pore size. These residues were then immersed in hexametaphosphoric acid and dispersed in the hexametaphosphoric acid using ultrasonic vibration. Here, the residuals in the hexametaphosphoric acid in which the residuals had been dispersed were captured using a new alumina filter with a pore size of 20 nm. The residuals captured on the alumina filter had a particle size of 20 nm or more. These captured residuals were decomposed with acid, and the Nb concentration was quantified in mass% using ICP atomic emission spectrometry. This was taken as the amount of Nb present in the residuals with a particle size of 20 nm or more.Here, the difference between the amount of Nb in all residuals and the amount of Nb present in residuals with a particle size of 20 nm or more was calculated, and this was used as the amount of Nb with a particle size of 20 nm or less (Nb. pre Next, the amount of dissolved Nb in the steel was calculated by calculating the difference between the total amount of Nb (Nb) contained in the steel and the amount of Nb in the total residual calculated by the above method. sol ) was decided. The Nb precipitates are mainly NbC, but may also contain NbN or other Nb precipitates.

[0091] The tensile test was carried out in accordance with JIS Z 2241 (2011) using a JIS No. 5 test piece, which was sampled so that the tensile direction was perpendicular to the rolling direction of the steel sheet, and TS (tensile strength) and EL (total elongation) were measured. In the examples, steel sheets with a TS of 980 MPa or more were judged to have achieved the target strength and passed. 1.5 Steel sheets with an xEl of 390,000 or more were judged to have achieved the target formability and passed the test.

[0092] The LME resistance was evaluated using an evaluation sample in which a sample measuring 100 mm in the direction perpendicular to the rolling direction and 30 mm in the rolling direction was taken from the steel plate and overlapped with a 980GA sample cut to the same size.

[0093] Resistance spot welding was performed on the evaluation samples with a stroke angle θ and a welding pressure of 3.5 kN. Here, the stroke angle in spot welding is defined as the angle θ between a line passing through the major axis of the nugget and a line parallel to the surface of the steel sheet in the cross section of the spot-welded part. The welding current pattern was controlled so that the resulting nugget diameter was in the range of 3.5√t to 5.5√t, where t is the thickness of a single steel sheet (1.2 mm). A Dr6-type CuCr electrode was used for resistance spot welding, with a 2.0 mm clearance between the overlapping evaluation samples and the electrode. The welding stroke angle and hold time for the LME resistance evaluation are shown in Tables 3-1 and 3-2.

[0094] For each evaluation, five evaluation samples were spot-welded to prepare welded components, and the cross section of the spot-welded components was observed using an optical microscope (magnification 100x) to evaluate their LME resistance. A case in which cracks occurred in two or fewer components and the average crack depth was less than 100 μm (including cases in which no cracks occurred) was rated as ⊚; a case in which cracks occurred in two or fewer components and the average crack depth was 100 μm or more but less than 300 μm was rated as ◯; a case in which cracks occurred in two or fewer components and the average crack depth was 300 μm or more was △; and a case in which cracks occurred in three or more components was ×. In the examples, steel sheets rated as ⊚, ◯, or △ were judged to have excellent LME properties and were designated as invention examples, while steel sheets rated as × were judged to have poor LME properties and were designated as comparative examples.

[0095] The high-strength steel sheets of the invention examples all have high TS and TS×El balance, and are excellent in LME resistance. On the other hand, the comparative examples are inferior in at least one of TS, TS×El balance, and LME resistance.

Claims

1. In mass %, C: 0.030% or more and 0.500% or less, Si: more than 0.01% and less than 2.50%, Mn: 0.10% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, O: 0.0100% or less, and Ti: 0.010% or more and 0.200% or less, Nb: 0.005% or more and 0.500% or less, and the balance being Fe and unavoidable impurities. Or, in mass %, C: 0.030% or more and 0.500% or less, Si: more than 0.01% and less than 2.50%, Mn: 0.10% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, O: 0.0100% or less, and Ti: 0.010% or more and 0.200% or less, Nb: 0.005% or more and 0.500% or less, Contains Furthermore, in mass % V: 0.500% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Co: 1.00% or less, Ni: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, Bi: 0.200% or less, and the balance being Fe and unavoidable impurities, The microstructure of the steel plate at the 1 / 4 position of the plate thickness is The total area ratio of tempered martensite and bainite is 40% or more and 85% or less, The area ratio of fresh martensite is 0% or more and 25% or less, The area ratio of retained austenite is 5% or more and 20% or less, The remainder is at least one of ferrite and pearlite in an area ratio of 0% to 20%. The Nb has a relationship among the amount of dissolved Nb (Nb sol ), the amount of Nb in Nb precipitates having a particle size of less than 20 nm (Nb pre ), and the total amount of Nb (Nb) contained in the steel sheet, which satisfies the following (Formula 1): A method for producing a high-strength steel plate having a diffusible hydrogen content of 0.50 mass ppm or less, The steel material having the above-described composition is heated to a temperature T ｓｏｌ a slab heating step of heating the slab at a temperature of 0.5 ° C. or higher for 1.0 hour or more; a hot rolling process; Winding temperature T ＣＴ a winding step in which the temperature is 650°C or less; Soaking temperature T ＡＴ The soaking temperature T is 750°C or higher and 950°C or lower. ＡＴ and an annealing step of holding the steel sheet at 200°C and cooling the steel sheet after holding the steel sheet, In the hot rolling process, the finish rolling start temperature T ＦＥＴ °C, finish rolling end temperature T ＦＤＴ °C, effective time t from the start of finish rolling to the end of finish rolling ＨＲ Q defined by (Equation 3) ＨＲ and, In the winding step, T ＦＤＴ Residence time t from ° C to 650 ° C ＣＴ , winding temperature T ＣＴ Q defined by (Equation 4) in °C ＣＴ and, In the annealing step, the soaking temperature T ＡＴ ℃, 650℃ to soaking temperature T ＡＴ Temperature rise time t to °C, soaking temperature T ＡＴ Holding time t in °C ＡＴ Q defined by (Equation 5) ＡＴ A method for manufacturing a high-strength steel plate that satisfies (Equation 6). (Formula 1) (Nb sol / Nb) + (Nb pre / Nb)≧0.40 In formula 1, Nb sol represents the amount of dissolved Nb (mass %), and Nb pre represents the amount of Nb (mass %) in Nb precipitates having a particle size of less than 20 nm. (Equation 2) T ｓｏｌ =7900 / (3.42-(log([Nb%][C%])))-273 Here, [Nb%] and [C%] are the amount of Nb (mass%) and the amount of C (mass%) contained in the steel, respectively. (Equation 3) Q ＨＲ =0.5 (T) ＦＥＴ +T ＦＤＴ )×log １０ (t) ＨＲ ) (Equation 4) Q ＣＴ =0.5 (T) ＣＴ +650)×log １０ (t) ＣＴ ) (Formula 5) Q ＡＴ = 0.5(650 + T ＡＴ ) × log １０ (t) + T ＡＴ × log １０ (t ＡＴ ) (Formula 6)Q ＨＲ +Q ＣＴ +Q ＡＴ ≦6000

2. Q ＡＴ 2. The method for producing a high strength steel plate according to claim 1, wherein the value of .gtoreq..times ...

3. The method for producing a high strength steel sheet according to claim 1 or 2, further comprising the step of: subjecting the steel sheet to a plating treatment after the annealing step.

4. The method for producing a high-strength steel sheet according to claim 3, wherein the plating treatment is an alloying plating treatment.

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